1. Field of the Invention
This invention relates to methods and systems for monitoring, detecting, and locating illicit cellular phone use, and more specifically relates to: such systems that are divided into multiple independent regions; such methods and systems capable of segregating cellular transmission from outside a specified area; methods of visually displaying such detections; methods of displaying historical records of logged detections; methods of estimating regions of uncertainty; methods that allow a sequence of time-discontinuous or frequency-hopping spread spectrum (FHSS) transmissions from a RF source to be identified as a single event; direct-conversion receivers (DCR's) used as radio frequency (RF) energy detectors and utilizing a bandpass intermediate frequency (IF) filter and methods relating to the same; methods of measuring peak signal levels of bursty RF sources, and more specifically relates to methods of accurately measuring peak signal levels of bursty RF sources across non-synchronized arrays of agile receivers; and DCR's having an integral variable attenuator capable of identifying potential harmonic distortion effects.
2. Description of the Prior Art
Very often secure facilities such as government buildings, court rooms, prisons and hospitals require restrictions of cellular phone use and communications within designated areas. The introduction of cell phones and other communications devices, whether intentional or accidental, can result in unauthorized disclosure of classified data, financial information, medical records, and the like. These devices also have the potential to electromagnetically interfere with the operation of critical equipment such as life support.
In prisons and other correctional facilities, it is essential to monitor and prevent illicit and unauthorized cellular communications. Illicit communications within these facilities create a safety hazard for the prisoners, employees, and surrounding citizens. Very often, cellular devices are smuggled into prisons so inmates may continue making illegal transactions and/or arrangements. Devices are hidden in mattresses, cells, and other common locations. Correctional facility employees often are bribed to import cellular devices for inmates. Conventionally, it is extremely difficult to track and monitor such devices and communications, as the common method is strictly a cell-by-cell search. The efficiency and effectiveness of this technique is limited due to the prisoners' and some employees' overwhelming desire to evade detection.
An effective solution must provide continuous, real-time coverage to ensure that new devices are detected as they are introduced into the facility. The system must not only detect the presence of unauthorized devices, but it must also determine their locations to allow prompt corrective action to be taken.
Facilities requiring monitoring include large and small facilities with multiple rooms and/or multiple levels constructed with a variety of techniques. Conventional systems measure RF source emissions within an array or grid of receivers placed in known positions. A single system monitoring a plurality of areas often can be inefficient and inaccurate. A system using measurements of RF propagation must provide the capacity to account for abrupt changes in RF propagation caused by obstructions such as floors, walls, doors, and certain furniture pieces. Emissions from RF sources also very often can leak through obstructions, creating false readings by the grid of receivers. A single array of receivers covering a facility is not effective or reliable; rather, a means of breaking the facility up into regions of consistent RF conditions is necessary, as is provided with the present invention. This ability provided with the present invention also provides a scalable means to efficiently accommodate systems with widely varying numbers of sensors and areas of interest.
Conventionally, individual sensors have been placed in areas of interest to detect phone signals in the vicinity of the region. Once detected, an alarm is triggered, alerting the proper authorities. Unfortunately, the conventional design is not capable of precisely detecting transmissions within a defined spatial area. Transmissions originating from cell phones or other mobile communication devices in close proximity to a restricted area, such as that from outside a shared door or the opposite side of a wall, trigger false alarms, as the calls actually are being placed from outside of the restricted region. This inability to differentiate between transmissions in restricted and unrestricted areas has caused the conventional design to be unreliable and inefficient.
Cell phones service is provided by a variety of networks operating at different frequencies and using a number of air interface protocols. An effective monitoring system, such as provided by the present invention, should be capable of detecting and locating devices using any of these protocols, networks, or frequencies. The ability to visually differentiate between frequency bands, as provided by the present invention, is an important tool to help authorities to determine that seized phones are consistent with the detected activity.
For evidentiary and investigative purposes, it is important for a cell phone monitoring system to provide a means to view a historical record of activity. Such a record is provided by the system of the present invention.
Any practical RF source location system will exhibit some amount of uncertainty or error in the locations determined for each source. As an aid to authorities to help determine the appropriate search area, an effective monitoring system should display information regarding the accuracy of the estimated location. Such information is provided by the system of the present invention.
Many times, cell phone calls or messages are divided into multiple RF bursts separated in time and/or frequency. Time-division-multiple-access (TDMA) schemes utilize a number of timeslots that are granted to different users; individual transmissions are broken up to fit into the allocated slots. Text, data, voice, and picture messages are often spread out into many RF bursts. Finally, cell phones may utilize frequency-hopping-spread-spectrum techniques to mitigate the effects of multipath fading or narrowband interference. An effective monitoring system should have the capability to aggregate these separated events to better represent the actual usage pattern. The system of the present invention has this capability.